1. Field of the Invention
This invention relates to a method and apparatus for precisely and non-destructively measuring an electrical characteristic of a material; and more particularly to measuring, at production-oriented speeds, properties such as propogation velocity factor and dielectric constant of thin film substrates at microwave frequencies.
Methods and apparatus for "production-oriented" testing advantageously have certain attributes which are different from the attributes of laboratory testing. For example, a production-oriented test technique should be capable of performance in a sufficiently short test time to be economically feasible. Of equal importance, or course, is that the production-oriented test be nondestructive. Another significant factor is that the test be sufficiently accurate, either in absolute accuracy or in calibratable repeatability. Also significant for a production-oriented test is that it provide measured values which indicate as directly as possible, without further computation, the desired characteristic which is being measured.
A particular testing problem arose from the following circumstances. It was known that electrical characteristics of microstrip circuits formed on thin film substrates, e.g., of alumina, are significantly dependent on two parameters, namely characteristic impedance and propogation velocity factor. These two parameters in turn are a function of the dimensions of the circuit applied to the substrate, the thickness of the substrate, and the dielectric constant of the substrate. One can readily calculate that for a substrate having a dielectric constant of about 10.0, a thickness of about 25 mils, and circuit line widths of about 25 mils, a 1 percent variation in either the line-width of the circuit applied to the substrate, the substrate thickness, or the dielectric constant will produce a 0.5% variation in characteristic impedance of the circuit applied to the substrate. Also, a one percent variation in either the line-width, the substrate thickness, or the dielectric constant will produce 0.36%, 0.05%, and 0.48% variation, respectively, in propogation velocity factor for a circuit formed on the substrate.
In view of the foregoing and in view of the tolerances which are often placed on microstrip circuits, a substrate is often required to meet a particular value of dielectric constant to within one percent. No production-oriented technique for measuring the dielectric constant or propogation velocity factor of substrates intended for use at microwave frequencies to the degree of accuracy is known.
2. Description of the Prior Art
One known technique for measuring dielectric constant with acceptable accuracy was reported by W. P. Harris and A. H. Scott in a report entitled "Precise Measurement of Dielectric Constant By The Two-Fluid Technique" in the Proceedings of The National Bureau of Standards Conference on Electrical Insulation, 1962, pages 51-53. This two-fluid technique involves doing four successive capacitance measurements at audio frequencies in two different fluids and then calculating the dielectric constant of the material from the capacitance measurements. This is not an acceptable production technique due to the considerable time required for the measurements and calculations and due to the complexity involved. Another problem with the two-fluid technique is that the dielectric constant of many materials of interest, e.g., aluminum oxide, varies with frequency. Thus, the effective dielectric constant at microwave frequencies is not the same as a measured value at an audio frequency.
Another known technique for measuring propogation velocity factor with acceptable accuracy was reported by H. F. Lenzing in a report entitled "Measurement of Dielectric Constant of Ceramic Substrates at Microwave Frequencies" presented at the Electronics Division, American Ceramic Society Meeting, Washington, D.C., May 10, 1972, Paper 41. The Lenzing technique consists of completely metallizing a substrate except for two very samll coupling slots and measuring the resonant frequencies of the rectangular cavity thus formed. The dielectric constant can then be calculated from the resonant frequencies measured and the length and width of the substrate. Unfortunately, this technique is both very time consuming and destructive, and so is not suitable for production-oriented testing.